It is well known that olefins such as ethylene, propylene and 1-butene can be polymerized in the presence of certain metallic catalysts, particularly the reaction products of organometallic compounds and transition metal compounds, to form substantially linear polymers of relatively high molecular weight. Typically such polymerizations are carried out at relatively low temperatures and pressures. The catalysts thus described produce polymers of narrow molecular weight distribution.
For many applications, such as extrusion and molding processes, it is highly desirable to have polymers which have a broad molecular weight distribution of the unimodal or the multimodal type. Such polymers exhibit excellent processability, i.e., they can be processed at a faster throughput rate with lower energy requirements with reduced melt flow perturbations.
It is also known to use certain organolanthanide compounds as olefin polymerization catalysts. Ballard et al in J.C.S. Chem. Comm., (1978) pages 994 and 995 reported that certain alkyl bridged complexes of lanthanide metals such as [(CH.sub.5 H.sub.5).sub.2 ErCH.sub.3 ].sub.2 and [(C.sub.5 H.sub.4 R).sub.2 YCH.sub.3 ].sub.2, would polymerize ethylene. Marks and Mauermann in their U.S. Pat. No. 4,668,773 disclose that an ethylene polymerization catalyst could be obtained by reacting a pentamethylcyclopentadienyl lanthanide halide ether complex with a special type of lithium alkyl and then reacting that product with hydrogen to yield a catalyst of the general [(C.sub.5 Me.sub.5).sub.3 LnH].sub.2. The dimeric hydrido complex of Marks et al was reported to have activities as high as 3,000 grams of polyethylene per millimole neodymium, however, the data also indicates that the catalyst had a very short lifetime (on the order of seconds to minutes). The overall productivity of the catalyst was thus quite low. The synthesis of Marks catalyst system was also very complex. It was a multi-step procedure that had to be carried out under rigorously anaerobic conditions. The procedure also requires the use of exotic organolithium compounds which contain no beta hydrogen or beta alkyl groups.